ApplyDeltaV:

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  Apply delta-v over a specified time interval with a simulation timestep

   Since version 7.
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  Computes the acceleration to be applied over the current time interval:

     t  ->  t+dT

  given the desired start and stop times, t0 and tF.

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   Usage:

   acc = ApplyDeltaV( t0, tF, t, dT, dV );  

     -or-

   acc  = ApplyDeltaV( mvr, t, dT );

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   ------
   Inputs
   ------
   t0            (1)   Desired start time for delta-v application
   tF            (1)   Desired finish time for delta-v application
   t             (1)   Current time [sec]
   dT            (1)   Time step for the simulation [sec]
   dV           (3,1)  Delta-v to be applied 

     -or-

   mvr           (.)   Maneuver data structure
   t             (1)   Current time [sec]
   dT            (1)   Time step for the simulation [sec]

   -------
   Outputs
   -------
   acc           (1)   Acceleration to apply over current time interval dT
   
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